Fabrication of magnetoresistive random-access memory (MRAM) devices normally involves a sequence of processing steps during which many layers of metals and dielectrics are deposited and then patterned to form a magnetoresistive stack as well as electrodes for electrical connections. To define the magnetic tunnel junctions (MTJ) in each MRAM device, precise patterning steps including photolithography and reactive ion etching (RIE), ion beam etching (IBE) or their combination are usually involved. During photolithography, patterns are transferred from a photomask to a light-sensitive photoresist, and later on transferred to MTJ stacks by RIE, IBE or their combination, forming separate and non-interacting MTJ devices. The hard mask during etch needs to be thick enough for excellent pattern integrity, especially for sub 60 nm devices.
One widely used low cost hybrid mask stack in the integrated circuit (IC) industry is composed of a thin (˜30 nm) spin-coated silicon hard mask on top of a thick (100-150 nm) spin-coated carbon hard mask. First, the thin silicon hard mask can be etched by fluorine carbon plasma, using a thin (lower than 200 nm) and high quality photoresist pattern mask. The thick carbon hard mask can then be etched by O2 based plasma which has a very low etch rate on the thin silicon hard mask above. To improve the future sub 60 nm MRAM device yield and variation, it is critical to enhance the process margin and one of the solutions is to thicken the carbon hard mask so that one can increase the thickness of the metal hard mask (top electrode) on top of the MTJ. This is important because a thick metal hard mask would provide enough protection for the later MTJ etch as well as better CMP polish control. However during spin-coating, the film thickness is controlled by revolutions per minute (RPM). Fewer RPM results in a thicker film, but the RPM cannot be too low, otherwise the film would be non-uniform. If carbon is directly spin-coated twice, part of the underneath carbon film would be dissolved, causing material loss and film quality degradation. A novel way to efficiently spin-coat a thick carbon hard mask is therefore needed.
Several patents teach using more than one hard mask layer including: U.S. Patent Applications 2007/0243707 (Manger et al) and 2016/0351791 (Zou et al) and U.S. Pat. No. 8,673,789 Kim) and U.S. Pat. No. 8,975,088 (Satoh et al). All of these references are different from the present disclosure.